From the definition of the sine function
... sin(51°) = 9/c
Solving for c gives
... c = 9/sin(51°) ≈ 11.58
From the definition of the tangent function
... tan(51°) = 9/b
Solving for b gives
... b = 9/tan(51°) ≈ 7.29
_____
Check
b² +9² = c²
7.29² +9² = 134.12 = 11.58² . . . . yes
_____
The mnemonic SOH CAH TOA can help you remember
... Sin = Opposite/Hypotenuse
... Cos = Adjacent/Hypotenuse
... Tan = Opposite/Adjacent